Switching arrangement with switching contacts and an inductive load

ABSTRACT

In the case of a circuit arrangement with switching contacts made from silver or a silver alloy for the purpose of switching an inductive load, for example a motor, in a direct-current circuit, the arcing time is adjusted as a function of the breaking current by appropriate design of the ratio of inductance and ohmic resistance in accordance with the prescribed relationship. It is possible as a result to prevent or at least minimize the material migration between the anode and cathode of the pair of switching contacts.

BACKGROUND OF THE INVENTION

The invention relates to a circuit arrangement with a pair of switchingcontacts made from silver and/or a silver alloy and with an inductiveload which can be connected to a DC voltage source via the pair ofswitching contacts.

Inductive loads in DC circuits occur, for example, in automobiles whereever more DC motors are being used owing to the increase in comfort andsafety. In particular, in safety systems, for example the anti-skidsystem, there is a need for a long lifetime in operating cycles, butalso for a high switching reliability, that is to say a low failurerate, in the range of use of the switching device, specifically therelay or switch. Both criteria, lifetime and failure rate, are stronglyinfluenced by the material migration of the contacts during switchingoperation under load in the case of direct current. Of importance inthis case is the formation of tips and holes on the contacts which, fromthe statistical point of view, are highly likely to lead to prematuremechanical sticking of the contacts.

The phenomenon of material migration has been known for a long time. Ithas already been proposed several times to keep this effect as slight aspossible by selecting specific alloys and specific pairs of contacts.Such silver alloys are described, for example, in EP 0 448 757 A1.However, in the case of selecting the contact materials purely from thepoint of view of material migration there is the risk that other contactproperties cannot be selected optimally.

Another known possibility of avoiding material migration consists inemploying circuit technology to intercept the arc by means ofspark-quenching elements. However, such a supplementary circuit iscostly.

SUMMARY OF THE INVENTION

The object of the invention is to employ circuit engineering in the caseof a circuit arrangement with switching contacts, made from silver or asilver alloy, and an inductive load so as to avoid material migration inthe simplest possible way without the contact material having to bespecially selected and without the need for expensive additional circuitelements in the load circuit.

According to the invention, this object is achieved when the arcing timeof the breakdown arc is determined by the circuit-specific time constantT from the ratio of the inductance L and the ohmic resistance R in theload circuit as a function of the predetermined design breaking currenti according to the following relationship: ##EQU1##

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the detailed description of thepresently preferred embodiments and from the drawings. ##EQU2##

Thus, the invention provides for the range, which is most interest inpractice, of the specific, expected breaking current from 1 to 30 A adesign rule for the load circuit, specifically for the ratio of theinductance to the ohmic resistance, and thus for the arcing time, bymeans of which the material migration can be completely suppressed or atleast greatly reduced. The invention makes use in this regard of thefollowing finding:

The material migration is the result of asymmetric evaporation processeson the two contacts, specifically the anode and the cathode. These areessentially produced by the breakdown arc, in particular in the case ofinductive loads. It is the arcing time of this breakdown arc which isdecisive in this regard. In certain regions of the arcing time, theasymmetry of the evaporation process can be compensated owing to specialphysical effects in such a way that the material migration virtuallyvanishes in the final analysis, that is to say no tips and holes areproduced on the contact surfaces. Such a relatively flat surface profileof the contacts decisively reduces the risk of mechanical sticking,lengthens the lifetime and increases the operating reliability of thecontacts.

Since the arcing time of the breakdown arc is essentially given by thetime constant, that is to say the cut-off inductance L of the inductivecomponent, for example a motor, and the ohmic resistance R thereof, itis possible according to the invention, for example, to select themotors in the load circuit in such a way that the breakdown arc assumesthe arcing time defined in accordance with the invention.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the detailed description of thepresently preferred embodiments and from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below using an exemplaryembodiment with the aid of a drawing, in which

FIG. 1 shows a simple circuit diagram of a circuit with an inductiveload, and

FIG. 2 shows a graph for the region, prescribed in accordance with theinvention, of the time constant for the breakdown arc as a function ofthe breaking current.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 shows a simplified circuit diagram of a load circuit with adirect-current source B, a switching contact S and an inductive loadwhich is represented by the series connection of an inductor L and anohmic resistor R. This load is preferably a DC motor as used, forexample, in automobiles for driving various functional units.

FIG. 2 shows, plotted on a logarithmic scale against the breakingcurrent i, the time constant T as the ratio of inductance L and ohmicresistance R, which determines the arcing time. In this case, theregions provided according to the invention for determining the timeconstant T are drawn in hatched in each case, specifically in the regionTB1 for breaking currents from 1 A to approximately 20 A and the regionTB2 for breaking currents between 20 A and 30 A. The regions each have aspecific band width, it being assumed in principle that in the case ofrelatively weak breaking currents the time constant should be situatedmore at the lower boundary of the respective region, and in the case ofrelatively strong breaking currents the time constant should be situatedmore at the upper boundary of the region. The exact values for optimumexclusion of the material migration can be determined in the individualcase for the contact materials employed by means of simple tests.

In the case of combinations of breaking currents and arcing times whichare situated in FIG. 2 to the left of and above the two optimum regionsTB1 and TB2, the material migration produces an anode gain, and in thecase of combinations to the right of and below these optimum regions,the material migration leads to a cathode gain.

The maximum upper limit of the arcing time of the arc should not exceed10 ms in the region TB2, since in the case of switching devices ofconventional design, that is to say in the case of relays and switches,from the statistical point of view the arc is no longer reliablyquenched above this arcing time given an open contact.

In this case, there is the risk that the contact system can be thermallydestroyed in a short time in the case of a single operating cycle. Inthe case of an additional electrical wiring of the switched load withthe aim of partial spark quenching, or in the case of wiring the drivecoil of the switching device in order to protect the drive electronics(resistor, diode, etc.), it is, of course, the value which is currentlyproduced and in this case, of course, depends on the load parameters (L,R) and the diverse wiring parameters, which is valid for the timeconstant. This can be detected using measurement techniques and beincluded in an optimized fashion in the stated favorable regionsaccording to the invention. It is therefore not necessary in this caseto quench the arc, something which could be relatively difficult andexpensive; instead, optimization with respect to its arcing time issufficient.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

The time constant (T) is fixed and invariable for selected values of Land R.

I claim:
 1. A circuit arrangement with a pair of switching contacts, andwith an inductive load connected to a DC voltage source via the pair ofswitching contacts, wherein an arcing time of a breakdown arc betweensaid contacts is determined by a time constant (T) from the ratio of theinductance (L) and the ohmic resistance (R) in the load circuit as afunction of the breaking current (i) according to the followingrelationship: ##EQU3##
 2. The circuit according to claim 1 wherein saidcontacts are made of silver.
 3. The circuit according to claim 1 whereinsaid contacts are made of a silver alloy.
 4. A circuit comprising:a pairof switching contacts which are connectable and disconnectable, abreakdown arc having a breaking current (i) occurring between saidcontacts upon disconnection thereof; an inductive load having a timeconstant (T), an inductance (L) and an ohmic resistance (R); and a DCvoltage source which is connectable across said load via said switchingcontacts; wherein: ##EQU4##
 5. The circuit according to claim 4 whereinsaid contacts are made of silver.
 6. The circuit according to claim 4wherein said contacts are made of a silver alloy.
 7. The circuitaccording to claim 4 wherein said inductive load is a motor.
 8. A methodof making a circuit having a pair of contacts which connect an inductiveload to a DC source, the inductive load having a time constant (T), aninductance (L) and a resistance (R), said contacts having a breakingcurrent (i), the method comprising the step of:selecting the inductiveload such that: ##EQU5##